Vehicles may be fitted with evaporative emission control systems such as onboard fuel vapor recovery systems. Such systems capture and prevent release of vaporized hydrocarbons to the atmosphere, for example fuel vapors generated in a vehicle gasoline tank during refueling. Specifically, the vaporized hydrocarbons (HCs) are stored in a fuel vapor canister packed with an adsorbent which adsorbs and stores the vapors. At a later time, when the engine is in operation, the evaporative emission control system allows the vapors to be purged into the engine intake manifold for use as fuel. The fuel vapor recovery system may include one more check valves, ejectors (or venturis), and/or controller actuatable valves for facilitating purge of stored vapors under boosted or non-boosted engine operation.
However, in some vehicles, including those that employ start/stop engines, conditions for purging the fuel vapor canister may be limited. In a first mode, when the fuel vapor canister effluent is rich, the purge flow is restricted via adjusting an opening of a canister purge valve to limit the fuel mass provided to the intake manifold of the engine by the fuel vapor canister. In a second mode, when the fuel vapor canister effluent is lean, flow through the canister and to the intake manifold may be restricted less (e.g., by opening the canister purge valve). However, the maximum total flow rate of air flowing through the fuel vapor canister and to the intake manifold may be limited due to reduced intake manifold vacuum during certain engine operating conditions, such as engine idle conditions.
One example approach for purging a fuel vapor canister during idle is shown by Orzel et al. in U.S. Pat. No. 5,215,055. Therein, a bypass throttle positioned in parallel with a main throttle is controlled based on a difference between an actual idle speed and desired idle speed. During purging of the fuel vapor canister, purge flow is reduced when the position of the bypass throttle is less than a preselected fraction of a maximum bypass throttle position.
However, the inventors herein have recognized potential issues with such systems. As one example, controlling purge flow from the canister and to the engine based on engine idle speed and a position of a bypass throttle may reduce the duration and opportunities for purging the fuel vapor canister. Additionally, decreasing an opening of a canister purge valve and/or bypass throttle in this way, based on engine idle speed, may result in lower intake manifold vacuum and decreased purging from the fuel vapor canister. As a result, vapors within the fuel vapor canister may build up over time and purging of the fuel vapor canister may be less efficient.
In one example, the issues described above may be addressed by a method for an engine, including: actuating a canister purge valve (CPV) to supply airflow to the engine via a fuel vapor canister while holding closed a main throttle and an auxiliary throttle arranged in parallel with the main throttle and in series with a venturi; and, as a desired intake manifold pressure increases, progressively opening the CPV, then the auxiliary throttle, and then the main throttle to achieve the desired intake manifold pressure. For example, the opening of the CPV may first be increased while holding the main throttle and auxiliary throttle closed to achieve the desired intake manifold pressure. If the desired intake manifold pressure cannot be obtained by fully opening the CPV alone, then the opening of the auxiliary throttle may be increased while holding the main throttle closed to achieve the desired intake manifold pressure. Similarly, if the desired intake manifold pressure cannot be obtained by fully opening the auxiliary throttle and the CPV, then the opening of the main throttle may be increased and modulated to deliver to the desired intake manifold pressure. In some embodiments, the desired intake manifold pressure may be a desired intake manifold vacuum during an engine idle condition. By holding the main throttle closed as long as possible during engine idle, jittering of the main throttle may be reduced, thereby reducing wear on the throttle position sensor. Additionally, by progressively opening the CPV, then the auxiliary throttle, and then the main throttle, air is first provided by the fuel vapor canister before being provided through the main intake air path. This may increase the frequency and amount of air being drawn through the fuel vapor canister, thereby purging the fuel vapor canister more frequently and maintain the effluent in the canister at a leaner state. By arranging a venturi in series with the auxiliary throttle and then opening the auxiliary throttle, fuel vapor purge vacuum may be increased, thereby allowing air to be continued to be drawn through the canister (whereas without this auxiliary throttle there may not be enough vacuum to continue drawing air through the canister). This progressive opening of the valves may also be performed during non-idle conditions, based on power demand from the engine, in order to deliver a desired air mass flow rate to the engine cylinders.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.